Planar and essentially rectangular fuel cell and fuel cell block

ABSTRACT

A planar, rectangular and water-cooled fuel cell includes a cooling element with a cooling chamber through which cooling water flows during the operation of the fuel cell. Cooling water does not flow through the cooling chamber in a homogeneous manner, normally resulting in local heating of the fuel cell in regions through which the cooling water flows through less frequently. A fuel cell is provided with a cooling element which includes an essentially rectangular cooling chamber with four corner regions, whereby the opening of the coolant flow is arranged in the first corner, the opening of a first coolant flow is arranged in a second corner and a second coolant flow is disposed in a third corner. The first coolant flow has a cross section Q 1  on the narrowest point thereof and the second coolant flow has a cross section Q 2  on the narrowest point thereof, the ratio of Q 1 /Q 2  being 7-25.

This application is the national phase under 35 U.S.C. § 371 of PCTInternational Application No. PCT/EP02/01911 which has an Internationalfiling date of Feb. 22, 2002, which designated the United States ofAmerica and which claims priority on European Patent Application numberEP 01105148.9 filed Mar. 2, 2001, the entire contents of which arehereby incorporated herein by reference.

FIELD OF THE INVENTION

The invention generally relates to a planar and essentially rectangularfuel cell having a cooling element. Preferably, the cooling element hasan essentially rectangular coolant space with four corner regions. Themouth of the coolant inflow is preferably arranged in a first cornerregion, and the mouth of a first coolant outflow is preferably arrangedin a second corner region. In addition, the invention generally relatesto a fuel cell block having such a fuel cell.

BACKGROUND OF THE INVENTION

In a fuel cell, electrical energy and heat are generated by thecombination of hydrogen (H2) and oxygen (O2) in an electrochemicalreaction, the hydrogen and the oxygen being combined to form water(H2O). A single fuel cell supplies an operating voltage of a maximum of1.1 V. For this reason, a plurality of planar fuel cells are stacked oneon top of the other and are combined to form a fuel cell block. Byvirtue of the fuel cells of the fuel cell block being connected inseries it is possible for the operating voltage of the fuel cell blockto be several hundred volts. A fuel cell in a fuel cell block includes adiaphragm electrode unit which is also referred to as an electrolyteelectrode unit, and the composite printed circuit board which isadjacent thereto on both sides. The composite printed circuit board canbe configured as cooling elements.

The technical implementation of the principle of the fuel cell has leadto different solutions, specifically with different types ofelectrolytes and operating temperatures between 80° C. and 1000° C.Depending on its operating temperature, the fuel cells are classified aslow-temperature fuel cells, medium-temperature fuel cells andhigh-temperature fuel cells which are distinguished in turn by varioustechnical embodiments.

The heat which is produced in a fuel cell by the electrochemicalreaction must be carried away from the fuel cell so that the fuel cellis not destroyed by overheating. In the case of a low-temperature fuelcell, this heat is usually carried away using a coolant circuit, thecoolant, generally water, flowing through the fuel cell, absorbing heatthere and giving off the heat outside the fuel cell. For this purpose,the fuel cell includes a cooling element which can be used either forcooling the fuel cell or else for heating the fuel cell, for examplewhen the fuel cell block is started up. The coolant element has acoolant space through which the coolant, generally the cooling water,flows while the fuel cell is operating. The coolant space has a coolantinflow and a coolant outflow, the coolant inflow and the coolant outflowbeing arranged in such a way that the stream of coolant which flows fromthe inflow to the outflow cools the fuel cell as uniformly as possible.

EP 0 591 800 B1 discloses a cooling element which is composed of twoplates and has a rectangular coolant space, the inflow and the outflowfor the coolant being arranged in corner regions of the coolant spacewhich are diagonally opposite one another. When cooling water flowsthrough such a coolant space, the centre region of the coolant space iseffectively cooled, but only a small amount of cooling water flowsthrough the corner regions of the coolant space which are not adjacentto the inflow or outflow. This results in the fuel cell being heated toa greater degree in these corner regions than in its central regionwhich adjoins the central region of the coolant space. In an extremecase, such defective conveying away of heat from the corner regionsthrough which there is a weak flow leads to the electrolyte diaphragm ofthe fuel cell being destroyed at these points.

SUMMARY OF THE INVENTION

An object of an embodiment of the present invention is therefore todisclose a fuel cell in which heat is conveyed away from the elements ofthe fuel cell which are adjacent to the coolant space in a homogenizedway in comparison with the prior art. In addition, an object of anembodiment of the present invention is to disclose a fuel cell blockwith a fuel cell with such improved conveying away of heat.

An object is achieved by a fuel cell which, according to an embodimentof the invention, has a second coolant outflow in a third corner regionof the coolant space. The first coolant outflow may have a flow crosssection Q1 at its narrowest point, and the second coolant outflow mayhave a flow cross section Q2 at its narrowest point, and the ratio Q1/Q2being 7 to 25.

By use of a second coolant outflow in a further corner region, animproved flow through this corner region which otherwise has a weakthrough-flow is achieved. As a result, the time for which the coolantwater is present in this corner region is reduced, enabling it to absorbmore heat there from the components of the fuel cell which are givingoff heat, and the conveying away of heat by the cooling water from thefuel cell is thus homogenized. The cooling element can be configuredwith only a single second cooling outflow in a corner region or elsewith two second coolant outflows in two different corner regions. Thesecond coolant outflow (or the second coolant outflows) is configured insuch a way that considerably less coolant can flow out of the coolantspace of the coolant element through said outflow than from the firstcoolant outflow. As a result, the main flow of coolant through thecoolant space from the coolant inflow to the first coolant outflow isnot significantly disrupted.

Only a small part of the coolant is branched off from this main flow anddirected through the second coolant outflow. This smaller amount isselected in such a way that it is sufficient to keep the corner region,through which there is otherwise a weak flow, at approximately the sametemperature as the central region of the fluid space.

It has been shown in trials that, with an essentially rectangularcoolant space, a coolant flow of approximately 3 to 10%, in particular 4to 7%, through the third corner region, that is to say through thesecond coolant outflow, is sufficient to cause heat to be conveyed awayuniformly in this corner region in comparison with the central region ofthe coolant space. Depending on the configuration of the coolantoutflows, such a flow to the second coolant outflow is achieved if theflow cross section Q1 of the first coolant outflow is approximately 7 to25 times as large as the flow cross section Q2 of the second coolantoutflow at its narrowest point. If the first coolant outflow isconfigured in the form of, for example, 20 individual small ducts, thesecond coolant outflow is expediently embodied in the form of, forexample, only one such duct. If the first coolant outflow is formed, forexample, from only a single duct, its flow cross section Q1 at itsnarrowest point is expediently 7 to 10 times the flow cross section Q2of the second coolant outflow which is embodied as a single duct.

In an advantageous configuration of an embodiment of the invention, thefirst and second corner regions are arranged essentially diagonallyopposite one another. The first corner region with the mouth of thecoolant inflow, and the second corner region with the mouth of the firstand large coolant outflow form the starting point and end point of themain flow of coolant through the coolant space of the cooling element ofthe fuel cell. If these two corner regions lie essentially diagonallyopposite one another, the largest possible quantity of heat istransferred from the fuel cell into the cooling water by this main flow.The regions of the coolant space through which this main flow flows tothe smallest degree are located in the two other corner regions of thecoolant space which are opposite one another. However, in one of thesecorner regions, or both of these corner regions, a second coolantoutflow is arranged through which the flow of coolant through thecoolant space is homogenized to a high degree.

In a fuel cell which is operated geodetically in an essentiallyvertically arranged fashion in a fuel cell block, that is to say in sucha way that the plane of the cells is oriented essentially perpendicularto the surface of the earth, air bubbles collect in the coolant space inthe course of the operation, at the upper edge of the coolant space. Inthe case of a fuel cell which is provided for such operation, the thirdcorner region with the second coolant outflow is expediently arranged atthe upper edge of the coolant space. In such an arrangement, the airbubbles can emerge from the coolant space through the second coolantoutflow, effectively avoiding overheating of the fuel cell at the upperedge of the coolant space.

An object which the fuel cell block can solve may be achieved by use ofa fuel cell block with a fuel cell in which the first coolant outflowopens into a first axial duct of the fuel cell block, and the secondcoolant outflow opens into a second axial duct of the fuel cell block,and the two axial ducts are connected to one another using a pressureequalizing line.

An axial duct is understood to be a duct which runs in the stackingdirection of the fuel cells within the fuel cell block which is composedof a plurality of stacked fuel cells. It is therefore oriented in theaxial direction of the fuel cell block. The cooling fluid is taken outof the fuel cell block through such an axial duct of the fuel cellblock. The coolant circuit in a fuel cell system which comprises a fuelcell block is generally an open circuit in which the pressure of thefluid within the axial duct which carries away cooling water isdependent on the geodetic height at which the axial duct, or a lineadjoining it, opens to atmospheric pressure.

The pressure ratio between the fluid pressure in the first axial ductwith respect to the fluid pressure in the second axial duct is thusdependent on where the two axial ducts, or a line which is connected tothem, open into the open air. As the flow of cooling fluid through acoolant outflow is dependent on the pressure within the axial duct towhich the cooling fluid opens, it is desirable for the fluid pressurewithin the first axial duct to be in a fixed ratio with respect to thefluid pressure within the second axial duct. This is because it is onlythis way that it is possible to ensure that the flow of fluid throughthe first coolant outflow is in a predeterminable ratio with respect tothe flow of coolant through the second coolant outflow. This ratio wouldthus be independent of the conveying of coolant from the fuel cell blockinto the fuel cell system.

As a result of a pressure equalization line between the two axial ducts,the pressure in the two axial ducts is essentially always the same. As aresult, the flow ratio through the two fluid outflows is always strictlydefined and independent of the opening of the axial ducts into the openair. Uniform conveying of heat out of the fuel cell into the coolant,and thus a uniform temperature within the fuel cell are thus achieved.

The pressure equalizing line can be embodied in the form of a line, butit can also be equally well formed by a duct in the fuel cell blockwhich connects the two axial ducts to one another. Such a duct may bearranged, for example, within the end plate or connecting plate of thefuel cell block or within an intermediate plate between the fuel cellblock and a humidifier which is adjacent to it.

BRIEF DESCRIPTION OF THE DRAWINGS

Exemplary embodiments of the invention are explained in more detail withreference to two figures, in which:

FIG. 1 shows a section through a cooling element of a planar andrectangular fuel cell;

FIG. 2 is an exploded diagram of a fuel cell block in a schematic view.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 represents a section through a cooling element 1 of a planar andrectangular fuel cell, the fuel cell including, in addition to thecooling element 1, a diaphragm electrode unit (not shown in FIG. 1)which is arranged underneath the cooling element 1 in terms of the viewin FIG. 1. The cooling element 1 includes a coolant space 3 and fourcorner regions 5 a, 5 b, 5 c, 5 d. The mouth of a coolant inflow 7 a isarranged in a first corner region 5 a, the coolant inflow 7 a connectingthe coolant space 3 to the axial duct 9 a. A first coolant outflow 7 b,which connects the coolant space 3 to a first axial duct 9 b, opens intoa second corner region 5 b. In a third corner region 5 c of the coolantspace 3 there is the mouth of a second coolant outflow 7 c whichconnects the coolant space 3 to a second axial duct 9 c. The coolantspace 3 has a fourth corner region 5 d to which, however, neither acoolant inflow nor a coolant outflow opens.

The fuel cell, and with it the cooling element 1, are planar in theplane of the paper of FIG. 1. The axial ducts 9 a, 9 b and 9 c runperpendicularly to the plane of the fuel cell, that is to sayperpendicularly to the plane of the paper. During the operation of thefuel cell, cooling fluid, for example water, flows out of a supplydevice assigned to the fuel cell and through the axial duct 9 a to thecooling element 1 of the fuel cell. It flows through the coolant inflow7 a and passes into the first corner region 5 a.

The greater part of the cooling water flows through the coolant space 3,flows through the second corner region 5 b and then the first coolantoutflow 7 b, and passes there to the first axial duct 9 b through whichit is directed away from the fuel cell. A small part of the coolingwater which passes through the coolant inflow 7 a into the coolant space3 flows along the upper edge 11 of the coolant space 3 and passes intothe third corner region 5 c. From there it flows through the secondcoolant outflow 7 c, passes into the second axial duct 9 c and is alsodirected away from the fuel cell through said axial duct 9 c. Airbubbles which collect in the coolant space 3 of the cooling element 1are driven through the effect of the gravitation to the upper edge 11 ofthe coolant space 3. This air is driven largely through the secondcoolant outflow 2 out of the coolant space 3 by the cooling water, andinto the axial duct 9 c from where it is expelled from the fuel cell.

The arrangement of the second coolant outflow 7 c in the corner region 5c ensures that the warm water which collects along the upper edge 11 isconveyed away. As a result, reaction heat which is generated in the fuelcell is given off uniformly to the cooling water within the coolingelement 1 in a regional fashion. Regional overheating of the fuel cellis thus effectively avoided.

The fuel cell, and with it the cooling element 1 are configured to beoperated arranged in a fuel cell block in such a way that the upper edge11 of the coolant space 3 is arranged at the top in terms of gravity. Asa result of this it is possible to dispense with a further coolantoutflow or inflow in the corner region 5 d. Cooling water which isheated in the lower half of the coolant space 3 is driven upwards byconvection and thus out of the corner region 5 d. As a result of this,there is a continuous flow of cool cooling water through the cornerregion 5 d. A third coolant outflow or a second coolant inflow in thecorner region 5 d is thus not necessarily required.

The coolant outflows 7 b and 7 c are each configured as a single ductwith a rectangular cross section. The flow cross section Q1 of the firstcoolant outflow 7 b has seven times the cross sectional area incomparison with the flow cross section Q2 of the second coolant outflow7 c. Due to the geometry of the coolant outflow 7 b and 7 c,approximately 7% of the cooling water which enters the coolant space 3through the coolant inflow 7 a flows through the second coolant outflow7 c.

In FIG. 2, three fuel cells 21 of a fuel cell block 22 are illustratedin the form of an exploded diagram. Each of these fuel cells 21 has acooling element 23 and a diaphragm electrolyte unit 25. The coolingelement 23 includes a frame 23 b which is joined on each of its twosides by a plate 23 a and 23 c, respectively. The frame 23 b thus forms,with the two plates 23 a, 23 c, a cavity, the coolant space.

Each of the fuel cells 21 has in each of the corners a triangularrecess. In fuel cells which are positioned one against the other, theserecesses form axial ducts 27 a, 27 b, 27 c and 27 d which run verticallywith respect to the plane of the cell. While the fuel cell block 22 isoperating, cooling water flows from a supply device (not shown in moredetail in FIG. 2) for the fuel cell block 22 into the inlet E of theaxial duct 27 a of the fuel cell block 22. The cooling water is directedthrough the axial duct 27 a to the cooling elements 23 of the fuel cellblock 22.

In each case some of the cooling water flows through the coolant inflow29 a of each cooling element 23 into the cooling space of the coolingelement 23. The greater part of the cooling water flows through thecoolant space in a diagonal direction and reaches the first coolantoutflow 29 b, through which it flows and reaches the first axial duct 27b. This cooling water from the cooling elements 23 of the fuel cells 21of the fuel cell block 22 collects in the first axial duct 27 b and iscarried out via the pressure equalization line 31, which is embodied asa line outside the fuel cell block 22, into the second axial duct 27 cin which it flows through the fuel cell block 22 and leaves it throughthe outlet A of the second axial duct 27 c. A small part of the coolingwater flows from the coolant inflow 29 a of each cooling element 23 tothe second coolant outflow 29 c, through which it is directed to asecond axial duct 27 c. There, it is combined with the cooling wateroriginating from the first axial duct 27 b, and flows to the outlet A ofthe second axial ducts 27 c.

The first coolant outflow 29 b of each coolant space is formed fromtwenty small ducts, only a few of which are shown in FIG. 2. The ductsconnect the coolant space to the axial duct 27 b. The second coolantoutflow 29 c of each coolant space is formed by a single duct whichconnects the coolant space to the second axial duct 27 c. The geometryof the ducts is in each case the same so that the flow cross section Q1,composed of twenty duct cross sections, of the first coolant outflow hastwenty times the area of the flow cross section Q2, composed of only oneduct cross section, of the second coolant outflow 29 c. The pressureequalization line 31 ensures that the fluid pressure within the axialducts 27 b and 27 c is essentially the same. The pressure ratios withinthe axial ducts 27 b and 27 c thus do not favor any of the flows fromthe coolant inflow 29 a to the coolant outflows 29 b and 29 c. Theamounts of coolant which flow off through the coolant outflow 29 b and29 c are thus determined decisively by the flow cross sections Q1 andQ2, so that approximately 5% of the cooling water flowing through thecoolant space leaves the coolant space through the second coolantoutflow 29 c.

The invention being thus described, it will be obvious that the same maybe varied in many ways. Such variations are not to be regarded as adeparture from the spirit and scope of the invention, and all suchmodifications as would be obvious to one skilled in the art are intendedto be included within the scope of the following claims.

1. A planar and essentially rectangular fuel cell, comprising: a coolingelement, including an essentially rectangular coolant space with fourcorner regions, the mouth of a coolant inflow being arranged in a firstcorner region, the mouth of a first coolant outflow being arranged in asecond corner region and a second coolant outflow being arranged in athird corner region of the coolant space, wherein the first coolantoutflow has a flow cross section Q1 at its narrowest point, the secondcoolant outflow has a flow cross section Q2 at its narrowest point, andthe ratio Q1/Q2 is 7 to
 25. 2. The fuel cell as claimed in claim 1,wherein the first corner region and the second corner region arearranged essentially diagonally opposite one another.
 3. A fuel cellblock comprising at least one fuel cell as claimed in claim
 1. 4. A fuelcell block comprising at least one fuel cell as claimed in claim
 2. 5. Afuel cell block as claimed in claim 3, wherein the first coolant outflowopens into a first axial duct of the fuel cell block, and the secondcoolant outflow opens into a second axial duct of the fuel cell block,and the two axial ducts are connected to one another using a pressureequalizing line.
 6. A fuel cell block as claimed in claim 4, wherein thefirst coolant outflow opens into a first axial duct of the fuel cellblock, and the second coolant outflow opens into a second axial duct ofthe fuel cell block, and the two axial ducts are connected to oneanother using a pressure equalizing line.
 7. A fuel cell, comprising: acooling element, including a coolant space with four corner regions, anopening of coolant inflow being arranged in a first corner region, afirst opening of coolant outflow being arranged in a second cornerregion, and a second opening of coolant outflow being arranged in athird corner region of the coolant space, wherein the first opening ofcoolant outflow including a flow cross section Q1 at its narrowestpoint, the second coolant outflow including a flow cross section Q2 atits narrowest point, and the ratio Q1/Q2 being approximately 7 to
 25. 8.The fuel cell as claimed in claim 7, wherein the first corner region andthe second corner region are arranged essentially diagonally oppositeone another.
 9. A fuel cell block comprising at least one fuel cell asclaimed in claim
 7. 10. A fuel cell block comprising at least one fuelcell as claimed in claim
 8. 11. A fuel cell block as claimed in claim 9,wherein the first opening of coolant outflow opens into a first axialduct of the fuel cell block, and the second opening of coolant outflowopens into a second axial duct of the fuel cell block, and the two axialducts are connected to one another using a pressure equalizing line. 12.A fuel cell block as claimed in claim 10, wherein the first opening ofcoolant outflow opens into a first axial duct of the fuel cell block,and the second opening of coolant outflow opens into a second axial ductof the fuel cell block, and the two axial ducts are connected to oneanother using a pressure equalizing line.